The present invention relates to a remotely installed transmitter capable of clearly picking up the speaker's voice despite being installed away from the speaker's mouth and to a hands-free two-way voice terminal device using such a transmitter.
In recent years, the use of portable telephones has increased tremendously. Many people carry portable telephones with them and use them in their cars while driving. Usually, however, the portable telephone must be held to use, which is not desirable from the standpoint of careful driving.
Therefore, a device has been developed with which a person can talk using a portable telephone but without using his hands. In this device, a transmitter is installed approximately 30 to 60 cm in front of the driver, such as on the sun visor of the car, and a speaker is installed under the driver's seat or in a similar place.
When the two-way simultaneous speech is conducted using such a transmitter and a speaker, however, the voice level of the speaker becomes less than about one-third that of the ordinary handset-type telephone, which picks up the voice signal close to the mouth, since the transmitter is located at a considerable distance from the mouth of the speaker. Therefore, ambient noise is likely to be picked up, making hands-free communication difficult in a noisy environment. To carry out two-way speech, furthermore, the signal from the speaker must be heard at the same time. Thus, unless the acoustic coupling is decreased between the transmitter and the speaker, feedback and echo occur.
Therefore, the above-mentioned device has been furnished with a circuit which lowers he sensitivity of the transmitter circuit at the time of reception and lowers the sound volume of the receiver circuit at the time of transmission. This prevents the occurrence of feedback and echo during the two-way communication.